Japanese Patent No. 3017953 discloses a rotor for an electric motor of the above-described type. FIGS. 8A and 8B of the present application illustrate the conventional rotor for an electric motor. As shown, the rotor includes a rotor core 102 provided for rotation around a stator 101. The rotor core 102 is fixed to a bed plate 103 made from a magnetic material. The rotor core 102 includes a plurality of magnet insertion sections 104 formed therein. A rotor magnet 105 is fixed to each magnet insertion section 104.
In the above-described construction, a position sensor 107 is disposed in an air-gap defined between the stator 101 and the rotor core 102. Accordingly, the position sensor 107 is subject to the influence of magnetic flux from the stator 101. Moreover, the rotor core 102 has an axial dimension or height set to the same value as the stator core 101. Accordingly, an amount of magnetic flux acting upon the position sensor 107 is so small that the positional detection by the position sensor is insufficient.
The solid line in FIG. 9 shows a quasi-sine wave of magnetic flux distribution in the air gap having the circumferential center of the rotor magnet 105 serving as the peak value. Two-dot chain line in FIG. 9 shows output of the position sensor 107. As obvious from FIG. 9, the detection accuracy of the position sensor 107 is low in the above-described construction such that the detection of normal quasi-sine wave is mistimed. As a result, since the control of the stator coil 108 is mistimed relative to the current position of the rotor core 102, oscillation or vibration occurs during operation of the motor and/or an operating efficiency is reduced.
Therefore, an object of the present invention is to provide a rotor for an electric motor which can improve the operating efficiency of the motor.
The present invention provides a rotor for an electric motor of an outer rotor type which includes a rotational shaft and a stator having teeth, the rotor comprising a base plate made of a magnetic material and coupled to the rotational shaft, a rotor core provided on the base plate and including a protrusion protruding axially relative to the base plate and the teeth of the stator, the rotor core having an outer peripheral face and an axial end, a plurality of magnet insertion portions defined in the rotor core, a plurality of rotor magnets provided in the magnet insertion portions respectively, each magnet having a stator; side and a counter-stator side both magnetized so as to have poles differing from each other, and a position sensor detecting a rotational position. of the rotor core and disposed radially or axially opposite the outer peripheral face or the axial end of the rotor core so as to correspond to the protrusion, wherein the rotor core has a plurality of slits formed therein so as to be located between the rotor magnets peripherally adjacent to each other, each slit having an open outer peripheral face.
In the above-described construction, the position sensor is disposed radially or axially opposite the outer peripheral face or the axial end of the rotor core. Thus, since the position sensor is located away from the stator, adverse effects of the stator to the position sensor can be restrained and the detecting timing of the position sensor can be accurate. Consequently, the operating efficiency of the motor can be improved.